In the Institute of Electrical and Electronics Engineers (IEEE) 802.16m System Description Document (SDD), structure of a frame is defined as a 20 ms superframe divided into four frames of 5 ms. Each superframe has a header at the beginning of the first frame. The SDD provides for two possible cyclic prefix (CP) lengths. The first length is ⅛ of the useful OFDM (Orthogonal Frequency-Division Multiplexing) symbol time, and the second length is 1/16 of the useful OFDM symbol time. The SDD is not explicit whether all four frames within a superframe must use the same CP length.